Method of synthesizing diglycerol tetranitrate, and solid rocket motor propellant containing the same

ABSTRACT

Diglyercol tetranitrate is an energetic nitrate ester plasticizer having no freezing point, making the nitrate ester plasticizer especially suited for use in solid rocket motor propellants that are subjected to low temperature storage and operational environments, which can reach as low as −54° C. in temperature. In order to avoid problems associated with fume-off that characterize the conventional synthesis method of making diglycerol tetranitrate, synthesis is performed in a medium including a mixed acid phase and an inert organic phase. The mixed acid phase contains, as ingredients, at least one nitronium ion source and at least one acid having sufficient strength to generate nitronium ions from the nitronium ion source. The nitronium ions in the mixed acid nitrate diglycerol to form diglycerol tetranitrate, which is then received into the organic liquid. The organic liquid, which preferably is a chlorocarbon such as dichloromethane, is insoluble with diglycerol but soluble with diglycerol tetranitrate. The inert organic phase is then treated to neutralize any acid contained in the inert organic phase, and the diglycerol tetranitrate is separated from the inert organic phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The benefit of priority is claimed based on U.S. ProvisionalApplication No. 60/191,548 filed in the U.S. Patent & Trademark Officeon Mar. 23, 2000, the complete disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a method of synthesizing diglyceroltetranitrate, which is an excellent plasticizer for energetic materialssuch as rocket motor propellants, explosives, and pyrotechnics. Thisinvention is also directed to a solid rocket motor propellant comprisingdiglycerol tetranitrate as the plasticizer.

[0004] 2. Description of the Related Art

[0005] Energetic materials used in solid rocket motor propellants,explosives, and pyrotechnics comprise an energetic material that, whenignited, releases sufficient amounts of energy to provide, in the caseof a propellant, the interior pressures needed to attain rocket motorflight or, in the case of an explosive, sufficient energy to demolish anintended target. Generally, energetic materials comprise, among otheringredients, a fuel and oxidizing agent immobilized in a polymericbinder. Selection of an appropriate binder can enhance the mechanicalproperties of the energetic material. Enhanced mechanical properties areimportant for maintaining the structural integrity of the energeticmaterials during operation and storage, especially when the energeticmaterials are subjected to operation and storage conditionscharacterized by extremely low and high temperatures. Other ingredientsare added to the composite solid propellant, as are needed or desired,to provide additional energy performance, improve the mechanicalproperties of the propellant, and/or simplify processing.

[0006] Among the additional ingredients commonly found in energeticmaterials are plasticizers. In particular, nitrate ester plasticizershave found wide acceptance as energetic plasticizers due to the abilityof nitrate ester plasticizers to enhance energetic performance. Nitrateester plasticizers provide the added benefits of improving rheologicalproperties during processing, preventing crystallization of the binder,and enhancing mechanical properties of the energetic material.

[0007] Due to the low temperatures at which energetic materials aresometimes stored, as well as the low temperatures that energeticmaterials existing as rocket motor propellants experience during highaltitude operation, military specifications sometimes require thatenergetic materials be resistant to prolonged exposure to temperaturesas low as −54° C. Inferior low temperature mechanical properties, suchas poor tensile strength and low strain capability, can generatemechanical strain in the energetic material at low temperatures and canpromote the likelihood of fracture to the energetic material. Forinstance, in the case of a solid propellant grain, fractures in thepropellant grain can, if widespread, significantly increase thepropellant grain surface area available for combustion reaction. As aconsequence, the chamber pressure created during combustion of apropellant grain can be increased to unanticipated levels, leading inextreme cases to catastrophic failure of the rocket motor in which thepropellant grain is loaded upon ignition.

[0008] The most commonly used conventional nitrate ester plasticizersreach their freezing points well above the military design specificationof −54° C. In particular, nitroglycerin has a freezing point of about−13° C. Butanetrioltrinitrate (BTTN) has a freezing point of −27° C. Thefreezing point of trimethylolethanetrinitrate (TMETN) is about −15° C.Upon freezing, these common nitrate ester plasticizers tend tocrystallize and migrate into agglomerations, disrupting the homogeneityof the energetic material and increasing the risk of fracture to theenergetic material.

[0009] As reported in Seymour M. Kaye, The Encyclopedia of Explosivesand Related Items (U.S. Army Armament Research & Development Command1983), between 1920 and World War II a currently less used nitrate esterplasticizer, diglycerol tetranitrate, was used in combination withnitroglycerin as a nitrate ester plasticizer for nitrocellulose-basedexplosives. Unlike nitroglycerin, BTTN, and TMETN, diglyceroltetranitrate does not have a freezing point, much less a freezing pointbelow −54° C. However, to the knowledge of the inventors, use ofdiglycerol tetranitrate has ceased and publications discussingdiglycerol tetranitrate have been few since approximately the end ofWorld War II. The cessation of activity relating to diglyceroltetranitrate is believed by the inventors to be attributable todrawbacks associated with the conventional synthesis method for makingdiglycerol tetranitrate. Conventional synthesis calls for the nitrationof diglycerol in a mixed acid comprising nitric acid to make diglyceroltetranitrate, followed by quenching of the mixed acid in water torecovery the diglycerol tetranitrate. However, a significant portion ofthe diglycerol tetranitrate formed in the mixed acid is not recovered byquenching the mixed acid. The difficulty in recovering diglyceroltetranitrate by quenching of a mixed acid is responsible not only forrelatively low yields of about 80 molar percent, as reported in the art,but also for fume-off problems. Residual nitric acid remaining in thespent mixed acid reacts with unrecovered diglycerol tetranitrate in anexothermic reaction that is autocatalytic. This exothermic reactiongenerates large amounts of nitrogen oxide and water in a process knownas a fume-off. Due to the autocatalytic nature of this reaction and theformation of large amount of nitrogen oxide, if left uncontrolled thefume-off can lead to violent explosion and other problems. Accordingly,great care in the handling and disposal of the waste acid is required toavoid unintentional explosion. Another problem of the conventionalsynthesis method stems from the solubility of the diglyceroltetranitrate in the mixed acid. During quenching an emulsion tends toform in the spent mixed acid. The emulsion is difficult and slow toseparate from the spent mixed acid, thus increasing process time and thelikelihood for fume-off.

[0010] It would, therefore, be a significant improvement in the art toprovide a method of synthesizing diglycerol tetranitrate in which therisk of fume-off is significantly reduced and which provides muchhigher, almost quantitative yields.

SUMMARY OF THE INVENTION

[0011] An object of this invention is to fulfill a long-felt need in theart by providing a method of synthesizing diglycerol tetranitrate thatattains the above-discussed improvement.

[0012] In accordance with the principles of this invention, the aboveand other objects are attained by nitrating diglycerol in a nitrating(or reaction) medium comprising a mixed acid phase and an inert organicphase. The mixed acid phase comprises, as ingredients, at least onenitronium ion source capable of nitrating each of the four hydroxylgroups of diglycerol and at least one acid having sufficient strength togenerate nitronium ions from the nitronium ion source. The inert organicphase comprises at least one organic liquid that is immiscible with themixed acid phase, so that the inert organic phase and mixed acid phaseare stratified. The organic liquid should neither dissolve nor nitratethe diglycerol. In practice, the inert organic phase provides a liquidmedium for receiving from the mixed acid phase the diglyceroltetranitrate generated by nitration of the diglycerol.

[0013] There are several advantages that are derived from practicing theinventive process. For example, the diglycerol tetranitrate has greatersolubility in the inert organic phase than the mixed acid phase, andupon synthesis migrates from the mixed acid phase to the inert organicphase. As a consequence, heat and diglycerol tetranitrate generatedduring the exothermic nitration reaction migrate from the mixed acidphase and into the inert organic phase, thereby reducing the likelihoodof fume-off and degradation of diglycerol tetranitrate in the mixed acidphase. Additionally, the migration of the diglycerol tetranitrate intothe inert organic phase simplifies and improves the efficiency ofdiglycerol tetranitrate recovery, thereby providing improved yieldscompared to the conventional synthesis route. For example, diglyceroltetranitrate yields are routinely on the order of 95% molar or greateraccording to the inventive method, and often approach or attainquantitative maximum yields. The spent mixed acid phase can then beneutralized and disposed of with lesser concerns over fume off and othersafety issues.

[0014] Synthesis by the method of this invention is particularly usefulfor making diglycerol tetranitrate for solid rocket motor propellants,especially tactical propellants designed for enduring storage andoperating temperatures as low as −54° C. Additionally, propellantscomprising diglycerol tetranitrate have significantly better safetyhandling properties compared to other energetic plasticizers, such asBTTN. In this regard, diglycerol tetranitrate exhibits a surprisinglylow sensitivity to ignition and detonation from accidental impact andshock stimuli. Diglycerol tetranitrate is also characterized by highviscosity and low vapor pressures, improving the handleability andprocessability of diglycerol tetranitrate in the production of solidrocket motor propellants.

[0015] Other objects, aspects, and advantages of this invention willbecome more apparent to those skilled in the art upon reading thespecification and appended claims which, when taken in conjunction withthe accompany drawing, explain the principles of this invention.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The accompanying drawing serves to elucidate the principles ofthis invention by illustrating a rocket motor assembly in which a solidpropellant comprising diglycerol tetranitrate plasticizer made inaccordance with the method of this invention may be loaded.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Diglycerol is a tetraol ether usually found as a mixture of threeisomers: 3,3′-oxy-di(1,2-propanediol), 2,2′-oxy-di(1,3-propanediol), and2-(hydroxymethyl)-3-oxyhexane-1,5,6-triol. As referred to herein,diglycerol includes one or a combination of more than one of theseisomers.

[0018] Among nitronium ion sources that can be used in accordance withthe present invention to nitrate the diglycerol are nitric acid andnitronium salts, such as nitronium nitrate and nitroniumtetrafluoroborate. The generation of nitronium ions from the nitroniumion source is performed with a strong acid. Although sulfuric acid ispreferred, other strong acids and anhydrides capable of generatingnitronium ions from the nitronium ion source, while being substantiallyinert with the nitronium ions, may be used as the strong acid inaddition to or as an alternative for sulfuric acid. Other acids that canbe used to generate nitronium ions from the nitronium ion source includemethane sulfonic acid (CH₃SO₂OH), acetic anhydride, acetic acid, andphosphoric acid, as well as combinations thereof.

[0019] The molar ratio of nitronium ion source (e.g., nitric acid) todiglycerol is at least a stoichiometric amount of 4:1 (four nitroniumions for the four hydroxyl groups of each diglycerol), but preferably isnot greater than about 10:1, more preferably 8:1, for economic reasons.

[0020] For practical reasons relating to availability and handleability,preferably nitric acid is used in combination with sulfuric acid to forma mixed acid phase. Various grades of nitric acid and sulfuric acid canbe used to make the mixed acid phase, the proportions of mixed acid(nitronium ion source and strong acid) to water in the mixed acid phasecan be from about 100:0 to about 80:20 by weight.

[0021] Prior to the addition of diglycerol to the mixed acid phase, themixed acid phase is combined with at least one organic liquid, which isimmiscible with the mixed acid and stratifies the mixed acid phase toform a separate inert organic phase. The inert organic phase preferablycomprises dichloromethane (also known as methylene chloride), althoughother chlorocarbons such as chloroform and dichloroethane may be usedalone or in combination with dichloromethane to provide the inertorganic phase. Further, the chlorocarbons of the inert organic phaseshould be inert and non-solvents with respect to the diglycerol reagent,yet should be capable of dissolving the diglycerol tetranitrate intosolution without degrading (or solvolyzing) the diglycerol tetranitrate.One of the advantages associated with the presence of the inert organicphase is that upon synthesis, the diglycerol tetranitrate product isremoved from the mixed acid phase as the product is received into theinert organic phase, thus eliminating the risks and hazards of fume-offthat would otherwise be associated with retention of the diglyceroltetranitrate in the mixed acid phase.

[0022] A sufficient volume of inert organic phase should be present topermit all of the diglycerol tetranitrate product to be received intothe inert organic phase. On the other hand, there is no upper limit onthe amount of inert organic phase, except as dictated by economicinefficiencies and waste management. The weight ratio of inert organicphase to diglycerol tetranitrate is preferably at least 1:1.

[0023] During the addition of the diglycerol to the nitrating medium,the temperature of the nitrating medium is preferably maintained in arange of from about 5° C. to about 20° C., more preferably 10° C. to 15°C. This may be accomplished, for example, by conducting the nitrationreaction in a jacketed reactor.

[0024] The inert organic phase containing the diglycerol tetranitratecan be separated from the mixed acid phase by liquid/liquid separationtechniques known in the art, including phase inversion, in which asufficient amount of water is added to the reaction medium to quench themixed acid phase and cause the inert organic phase to become denser thanthe mixed acid phase. Separation funnels can be used to separate theinverted phases.

[0025] Any residual acid in the separated inert organic phase can thenbe neutralized by addition of one or more suitable neutralizationagents, typically in the form of a weak base or weak bases.Representative neutralization agents include: carbonates, such as sodiumcarbonate and potassium carbonate, and calcium carbonate; andbicarbonates, such as sodium bicarbonate, and potassium bicarbonate.

[0026] The glycerol tetranitrate can then be separated from the organicliquid (e.g., methylene chloride), for example, by evaporation. In thecase of the use of methylene chloride, evaporation can be performedunder reduced pressures at temperatures of about 30° C.

[0027] The diglycerol tetranitrate is especially useful as a plasticizerfor solid rocket motor propellants, including elastomer-based compositepropellants, modified composite propellants, crosslinked double-basepropellants, and other plasticized propellants.

[0028] Representative nitrate ester plasticizers that optionally can beused in combination with the diglycerol tetranitrate to furtherplasticize the energetic composition include, by way of example,nitroglycerine, trimethylolethanetrinitrate, triethyleneglycoldinitrate,diethyleneglycol-dinitrate, ethyleneglycol dinitrate,butanetrioltrinitrate, alkyl NENA's, or combinations thereof. Thepropellant can also include one or more inert plasticizers, such astriacetin (glycerol triacetate), dioctyladipate, isodecylperlargonate,dioctylphthalate, dioctylmaleate, dibutylphthalate, di-n-propyl adipate,diethylphthalate, dipropylphthalate, CITROFLEX®, diethyl suberate,diethyl sebacate, diethyl pimelate, or combinations thereof.

[0029] Solid rocket motor propellants also commonly include inorganicoxidizers and metal fuels. Representative inorganic oxidizers include,by way of example, ammonium perchlorate, ammonium nitrate,hydroxylammonium nitrate, ammonium dinitramide, potassium dinitramide,potassium perchlorate, or combinations thereof. Representative fuelsinclude metals, such as aluminum, magnesium, boron, titanium, silicon,and alloys and/or mixtures thereof. The fuel and oxidizer may be presentas powder, or in particulate or other forms. Other ingredients known inthe art that optionally can be included in the solid propellant invarious combinations include the following: bonding agents such asTEPANOL; energetic fillers such as nitramines; thermal stabilizers suchas N-methyl-p-nitroaniline; ballistic modifiers such as titaniumdioxide, lead compounds, and bismuth compounds; reinforcing fibers; andpressure oscillation stabilizers, such as zirconium carbide and alumina.Methods of making and casting solid rocket motor propellants, as well asacceptable combinations and concentrations of ingredients, are withinthe purview of those skilled in the art of rocket motor science.

[0030] An example of a rocket motor assembly comprising a solid rocketmotor propellant containing diglycerol tetranitrate is shown in theFIGURE, in which the rocket motor assembly is generally designated byreference numeral 10. The assembly 10 includes a solid propellant grain12 loaded within the interior surface of the rocket motor case 14.Typically, insulation 16 and a liner 18 are interposed between the case14 and the solid propellant grain 12. The insulation 16 and the liner 18serve to protect the case from the extreme conditions produced duringcombustion of the solid propellant grain 12. Methods for loading arocket motor case 14 with the insulation 16, the liner 18, and the solidpropellant grain 12 are known to those skilled in the art, and can bereadily adapted without undue experimentation to incorporate thepropellant of this invention. Liner compositions and methods forapplying liners into a rocket motor case are also well known in the art.Also shown in the FIGURE is an igniter 20 attached to the forward end ofthe case 14 for igniting the solid propellant grain 12 and a nozzleassembly 22 attached at the aft end of the case 14 for expelling at highvelocities combustion products generated during burning of the solidpropellant grain 12.

[0031] The following examples are offered to further illustrate thesynthesis methods of the present invention. The examples are intended tobe exemplary and should not be viewed as exhaustive of the scope of theinvention.

EXAMPLES Example 1

[0032] Sulfuric acid (96%, 20 ml) was added to nitric acid (90%, 20 ml),then cooled to below 38° C. before adding methylene chloride (100grams). This mixture was cooled to 0° C. in an ice bath and 8 grams ofdiglycerol were added dropwise over 0.5 hour. At this temperature of 0°C., the diglycerol tended to coagulate despite rapid agitation of thereaction mixture. After another 0.5 hour the coagulated diglycerol haddisappeared and the reaction mixture was poured onto 40 ml of crushedice, which dissolved to form a dilute acid. The organic phase wasseparated from the dilute acid with a separation funnel and was washedwith 50 ml of saturated sodium bicarbonate solution. The organic phasewas then dried over 5 grams of magnesium sulfate, filtered, and thevolatiles were removed at 30° C. under reduced pressure to givediglycerol tetranitrate as a pale yellow oil in 95% yield.

Example 2

[0033] Sulfuric acid (96%, 30 ml) was added to nitric acid (90%, 30 ml),then cooled to below 38° C. before adding 100 ml of methylene chloride.This mixture was further cooled to 10° C. and 12 g of diglycerol wereadded by pouring in a steady thin stream over 15 minutes. The rate ofaddition was such that with a cooling bath at 0° C. the reactiontemperature stayed between 10° C. and 12° C. Under these conditions thediglycerol did not coagulate. After a further 15 minutes the reactionmixture was poured onto 200 ml of crushed ice to dissolve the ice andform a dilute acid. The organic phase was separated from the dilute acidwith a separation funnel and was washed with 50 ml of saturated sodiumbicarbonate solution. The organic phase was then dried over 5 grams ofmagnesium sulfate, filtered, and the volatiles were removed at 30° C.under reduced pressure to give diglycerol tetranitrate as a pale yellowoil in 96% yield.

Example 3

[0034] Sulfuric acid (96%, 120 ml) was added to nitric acid (90%, 120ml), then cooled to below 38° C. before adding 400 ml of methylenechloride. This mixture was cooled to 5° C. and 48.4 grams of diglycerolwere added by pouring in a steady thin stream over 15 minutes. The rateof addition was such that with a cooling bath at 0° C. the reactiontemperature stayed at 5° C. Under these conditions the diglycerol didnot coagulate, but the diglycerol did tend to form a film on the reactorsides and thermometer before it reacted and dissolved. After a further15 minutes the reaction mixture was poured onto 1 liter of crushed iceto form a dilute acid. The organic phase was separated from the diluteacid with a separation funnel and was washed twice with 500 ml ofsaturated sodium bicarbonate solution. The organic phase was then driedover 5 grams of magnesium sulfate, filtered, and the volatiles wereremoved at 30° C. under reduced pressure to give diglycerol tetranitrateas a pale yellow oil in 96% yield.

Example 4

[0035] Sulfuric acid (96%, 1200 ml) was added to nitric acid (90%, 1200ml), then cooled to below 38° C. before adding 3000 ml of methylenechloride. This mixture was cooled to 10° C. and 460 g of diglycerol wereadded by pouring in a steady thin stream over 65 minutes. The rate ofaddition was such that with a cooling bath at 0° C. the reactiontemperature stayed between 10° C. and 15° C. Under these conditions thediglycerol did not coagulate. After a further 30 minutes the organicphase was separated from the acid phase without dilution. The organicphase was washed twice with 500 ml of saturated sodium bicarbonatesolution. The organic phase was then dried over 5 grams of magnesiumsulfate, filtered, and the volatiles were removed at 30° C. underreduced pressure to give diglycerol tetranitrate as a pale yellow oil in97.3% yield.

[0036] The foregoing detailed description of the invention has beenprovided for the purpose of explaining the principles of the inventionand its practical application, thereby enabling others skilled in theart to understand the invention for various embodiments and with variousmodifications as are suited to the particular use contemplated. Thisdescription is not intended to be exhaustive or to limit the inventionto the precise embodiments disclosed. Modifications and equivalents willbe apparent to practitioners skilled in this art and are encompassedwithin the spirit and scope of the appended claims.

What is claimed is:
 1. A method of synthesizing diglycerol tetranitrate, comprising: forming a nitrating medium comprising a mixed acid phase and an inert organic phase, the mixed acid phase comprising, as ingredients, at least one nitronium ion source and at least one acid having sufficient strength to generate nitronium ions from the nitronium ion source, the inert organic phase comprising at least one organic liquid in which diglycerol is insoluble yet in which diglycerol tetranitrate is soluble; combining the nitrating medium with diglycerol and nitrating the diglycerol in the mixed acid phase to form diglycerol tetranitrate, which is received into the inert organic phase; separating the inert organic phase having the diglycerol tetranitrate dissolved therein from the mixed acid phase; and recovering the diglycerol tetranitrate from the inert organic phase.
 2. The method of claim 1, further comprising neutralizing any of the acid or nitronium ion source present in the inert organic phase subsequent to said separating of the inert organic phase from the mixed acid phase.
 3. The method of claim 1, wherein the nitronium ion source comprises nitric acid and further wherein the acid comprises sulfuric acid.
 4. The method of claim 3, wherein a molar ratio of the nitronium ion source to the diglycerol is at least 4:1 and not greater than about 8:1.
 5. The method of claim 3, wherein a weight ratio of the nitric and sulfuric acids to water in the mixed acid phase is from about 100:0 to about 80:20.
 6. The method of claim 3, wherein the organic liquid comprises at least one organic chlorocarbon.
 7. The method of claim 6, wherein the chlorocarbon is dichloromethane.
 8. The method of claim 3, wherein a weight ratio of the inert organic phase to the diglycerol tetranitrate is preferably at least 1:1.
 9. The method of claim 3, wherein said nitrating of the diglycerol further comprises maintaining the nitrating medium in a range of from about 5° C. to about 20° C.
 10. The method of claim 3, further comprising neutralizing any of the acid or nitronium ion source present in the inert organic phase subsequent to said separating of the inert organic phase from the mixed acid phase.
 11. A rocket motor assembly comprising a rocket motor case, a solid propellant grain loaded in the rocket motor case, and a nozzle in operative association with the rocket motor case to receive and discharge combustion products generated upon ignition of the solid propellant grain, the solid propellant grain comprising diglycerol tetranitrate plasticizer.
 12. The rocket motor assembly of claim 11, wherein the diglycerol tetranitrate plasticizer is synthesizing by a method comprising: forming a nitrating medium comprising a mixed acid phase and an inert organic phase, the mixed acid phase comprising, as ingredients, at least one nitronium ion source and at least one acid having sufficient strength to generate nitronium ions from the nitronium ion source, the inert organic phase comprising at least one organic liquid in which diglycerol is insoluble yet in which diglycerol tetranitrate is soluble; combining the nitrating medium with diglycerol and nitrating the diglycerol in the mixed acid phase to form diglycerol tetranitrate, which is received into the inert organic phase; separating the inert organic phase having the diglycerol tetranitrate dissolved therein from the mixed acid phase; and recovering the diglycerol tetranitrate from the inert organic phase. 